1. Field of the Invention
The invention relates to electric lamps and particularly to high intensity arc discharge electric lamps. More particularly the invention is concerned with ceramic high intensity arc discharge lamps with pure metal fills.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Ceramic high intensity arc discharge lamps are a good source of intense white light, and are convenient for projectors and other beam producing fixtures. They are commonly made with a ceramic main body that may be cylindrical or bulbous and have two axially extending elongated capillaries supporting the sealed leads. Capillaries typically have a length to diameter ratio of 10 or more. The long capillaries provide a large temperature gradient between the hot interior end near the main body and the cooler exterior end near the capillary tip. A metal electrode, typically an extended rod assembly of a tungsten electrode tip, an extension section which may be molybdenum or cermet and a sealing section commonly niobium, may then be frit sealed along the niobium portion to the cooler end of the capillary. The elongated capillaries necessarily form axially long lamps that are difficult to position in small volume fixtures such as small projectors. There is then a need for ceramic discharge lamp without capillaries or capillary seals.
In operation, the seal temperatures of an HID lamp must be maintained below the melting temperature of the weakest element. Typically the weakest element is the frit seal. The frit is kept cool by extending the seal away from the main ceramic body by the long capillary. The maximum operating temperature of the frit frequently sets the cold spot temperature of the lamp, thereby limiting the materials that can be vaporized in the lamp during operation. There is then a need for a lamp with a higher operating seal temperature.
Heat flow along the capillary is thermally resisted by using a narrow cross section and by radiating heat, convectively cooling or otherwise loosing heat over the extended capillary length. There are several problems with cooling the electrode over an extended capillary to preserve the frit. The first is the capillaries extend the size of the lamp, limiting its positioning in small fixtures. A second problem is that the heat lost in the electrode cooling is really energy lost from light production. The heat loss also lengthens the start-up time from ignition to the full on state. There is then a need for a lamp with a hot seal.
The residual volume surrounding the electrode assembly in the capillary acts as a reservoir for the fill materials. This reservoir can disproportionately hold or supply fill materials to the discharge or can provide a reaction zone generating undesirable compounds interacting with the discharge, the electrode assembly or the envelope wall. Salts which enter and leave the residual volume in an uncontrolled manner may cause to time varying color shifts. There is a need to reduce or eliminate the residual volume in the seal region of a discharge lamp, and thereby limit such effects.
Pure metals are generally more reactive than are the iodide salts commonly used in a high intensity discharge lamp, and would therefore normally cause problems with frit seal materials. It is an object of the invention to enable a seal tolerant of pure metal fills.